1. Field of the Invention
This invention relates to photographic silver halide materials and systems. More particularly, this invention relates to materials useful for preparing color separation films. Still more particularly, this invention relates to color separation films with color gradients that have a particular relationship or balance to each other. Finally, this invention relates to color separation films wherein the balanced color gradients are achieved under indentical processing conditions.
2. Description of the prior Art
The first step in preparing a printing plate for the multiple reproduction of a colored original involves the photographic recording of the original on so-called "color separation films". In this process, the original image is successively photographed three or more times, each time through a different color (red, green and blue) filter on a new piece of the color separation film. These films are then processed and separate printing plates made therefrom. The plate made from the red filter separation is used to print with cyan ink, the plate from the green filter separation is used to print with magenta ink, and that from the blue filter separation is used to print with yellow ink. When a four-color printing is made, a black filter separation is also made and used to print with black ink. These plates are then used, for example, in offset lithography as taught by Jaffe et al. in "Color Separation Photography", Graphic Arts Technical Foundation, Inc., August 1959. Preparation of the photographic copy, conversion of the color negative image to the color printer plate and the associated systems are fully covered therein. Other processes which use the color separation film are letterpress gravure printing and stencil or screen printing as described by Yule in "Principles of Color Reproduction", John Wiley and Sons (1967). To produce the satisfactory gray balance in the final reproduction, most cyan inks require a slightly higher contrast in the cyan printer (from the red separation film) than in the other printers. Contrast or gradient herein refers to the slope of the sensitometric curve produced from a step-wise image exposure of the film. Ideally, all contrasts should be equal. However, as stated above, it is preferred to have the green and blue contrasts equal but slightly lower than the red to achieve the desired gray balance described above.
In practice, the operator adjusts the developing conditions to achieve the desired gradient balance. Where machines are used to process the photographic element and where film throughput is important, variations in processing time and temperature by adjustment of the machine controls is cumbersome and time-consuming. Additionally, the operator of the machine finds it difficult to exactly adjust the machine parameter from set to set, resulting in reproducibility errors.
Another method for adjusting the color gradient balance is to incorporate a removable light-absorbing filter dye within the emulsion which will artificially lower a particular color contrast by absorbing light in the region of the spectrum to which the color exposure contrast is too high. Such a system is described by Salesin in U.S. Pat. No. 3,531,287. Other effects are noted, however, such as a large loss in emulsion speed. Additionally, these filter dyes must be totally removable in the processing fluids else they will stain the final record.
It is further known to add a second emulsion layer which has not been optically sensitized in order to raise the blue exposure contrast. This results in a product wherein all of the color contrasts are almost equal as taught by Hellmig in U.S. Pat. No. 3,396,024. However, equal color gradients are not desired by the industry that uses the color separation films, because they cannot achieve the proper tonal relationship.